(a) Field of the Invention
The present invention relates to a bidirectional switching circuit, more particularly a switching circuit for triggering a bidirectional thyristor such as TRIAC which is a trademark of a product of General Electric, Inc. of U.S.A.
(b) Description of the Prior Art
A bidirectional thyristor, in general, when employed as, for example, a power controlling element of a power supply circuit requires a switching circuit as a triggering element or a circuit for triggering this bidirectional thyristor. As the triggering elements of this type, there have been proposed and placed on the market various elements such as DIAC (Diode AC Switch), SBS (Silicon Bilateral Switch) and PUT (Programmable Uni-Junction Transistor). Among them, a DIAC is arranged to be operative so that, for example, in such circuit as shown in FIG. 1, when a voltage which is applied to a DIAC 4 across terminals 1 and 2 and via a resistor 3 has reached a certain value (usually about 30 V), the electric charge which has been stored in a capacitor 5 is caused to flow to a gate of a bidirectional thyristor 6 to render this thyristor 6 conductive. And, by the use of this circuit, it is possible to control the firing angle of the bidirectional thyristor 6 by varying the resistance value of the resistor 3.
Also, as SBS may be expressed by its equivalent circuit which is of such arrangement as shown in FIG. 2A. For example, in a circuit shown in FIG. 2B (which is a circuit as shown in FIG. 1 wherein the DIAC 4 is substituted by an SBS 7), the SBS is designed to be operative so that, when a voltage applied to the SBS 7 has reached a certain voltage level (usually about 8 V), the bidirectional thyristor 6 is rendered conductive in a manner same as that performed by the aforesaid DIAC.
Furthermore, a PUT may be expressed by an equivalent circuit which is arranged as shown in FIG. 3A. This PUT is used in a manner as, for example, shown in FIG. 3B. This circuit shown in FIG. 3B is intended, in a circuit for supplying to a load 23 an ac voltage applied across terminals 21 and 22, to trigger by a PUT 25 a bidirectional thyristor 24 which is inserted between the terminal 22 and the load 23. Since this PUT is a uni-directional element, this circuit is provided with a rectifying circuit 26 for driving the PUT 25 and a driving circuit 28 which is comprised of a Zener diode 27 and other elements. This circuit is arranged to be operative so that, when the PUT 25 is rendered conductive, an electric current is allowed to flow to the primary side of a transformer 29, and that the bidirectional thyristor 24 is triggered by a voltage induced on the secondary side of the transformer 29.
Among those elements described above, the DIAC and the SBS have the inconveniences such that, when these elements are employed in such circuits as shown in FIG. 1 and FIG. 2B, the charge-up time of the capacitor 5 varies in accordance with the frequency of the voltage applied across the terminals 1 and 2, so that the timing at which the DIAC 4 or the SBS 7 is rendered conductive will vary in accordance with this frequency. Thus, there is the drawback that the values of the resistor 3 and the capacitor 5 require to be varied in accordance with the frequency of the voltage which is applied. More particularly, in the circuits shown in FIG. 1 and FIG. 2B wherein these elements are employed, if the values of the resistor 3 and the capacitor 5 are set in such way that, for example, these circuits are actuated by a commercial power supply with a frequency of 50 Hz, there could happen that the timing for rendering the DIAC 4 or the SBS 7 conductive is influenced by the power supply frequency in such manner that there arises an instances wherein the circuits are not actuated by a commercial power supply with a frequency of 60 Hz. Also, when such circuitry is connected to a power supply, if a commercial power is applied, for example, at half-way of a half cycle of the power which is supplied, the charge-up of the capacitor 5 will be commenced exactly at the time the commercial power is applied. Therefore, there will arise an instance wherein there is not performed a charge-up of the capacitor 5 to an extent enough for rendering either the DIAC 4 or the SBS 7 conductive before the termination of this half cycle. Such operations will constitute factors to make the set firing angle unstable.
Also, in case the circuit shown in FIG. 1 or FIG. 2B is a circuit intended for controlling the power of the power supply circuit, i.e. in case it is a circuit arranged so that the bidirectional thyristor 6 shown in these Figures is inserted in a power supply line for a load, and that the resistor 3 is comprised of a variable resistor, so as to be operative that the fluctuation of the voltage applied to the load is detected to control the value of this resistor 3 to compensate for said fluctuation of the voltage to thereby control the firing angle of the bidirectional thyristor 6, there will arise the following problem. That is, because the controlling of the resistor 3 is performed only when the bidirectional thyristor 6 is rendered conductive at the time the circuit is connected to the power supply, there would arise an instance wherein the actuation of the bidirectional thyristor 6 is not carried out at all depending on the condition of the resistor 3 at the time the circuit is connected to the power supply, i.e. an instance wherein the value of the resistor 3 is extremely large so that the capacitor 5 is not charged up. Thus, there is the drawback that the actuation of the bidirectional thyristor 6 becomes unstable. Furthermore, such known arrangement, in conjunction with the inconvenience that the timing at which the bidirectional thyristor 6 is rendered conductive would vary depending on the difference in the power supply frequency such as 50 Hz and 60 Hz, would make the actuation of the bidirectional thyristor 6 all the more unstable.
Also, the PUT, when used in such manner as shown in FIG. 3B, is able to eliminate the drawback and inconvenience of the aforesaid DIAC and SBS. On the other hand, however, the PUT has no bidirectional characteristic, so that there is required a driving circuit 28 having such complicated arrangement as shown in FIG. 3B, and also there is required a transformer 29. Thus, the PUT has the drawback that the resulting circuit becomes costly.